Apparatus and method for squeeze casting

ABSTRACT

Squeeze casting of high integrity, near net shape castings is performed in a mould cavity defined by cooperating die parts which are movable with respect to each other and have a separation distance which is selected to define a predetermined cavity volume for the cast article. A conduit has a first end connected to an entrance in the lower die part of the mould cavity and a second end connected to a receptacle which contains molten metal. Molten metal is transferred upwardly from the receptacle through the conduit to fill or substantially fill the mould cavity. The entrance to the mould cavity is sealed, and pressure is applied on the die parts to further reduce the cavity volume during solidification of the metal in the mould cavity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 of PCT/GB96/00137, filed on Jan. 23, 1996.

The present invention relates to an apparatus and method for squeezecasting.

Two types of squeeze casting are known in the prior art end are referredto as direct squeeze casting and indirect squeeze casting. Bothprocesses have been developed for the production of high integrity nearnet shape castings. However, in order to produce important castingconditions need to be satisfied:

(i) delivery of clean metal to the mould, usually involving filtration;

(ii) non-turbulent flow of metal into the die set after filtration;

(iii) accurate metering of metal into the die cavity;

(iv) full pressurisation of the metal during solidification.

When measured against these criteria, serious drawbacks and limitationsare apparent in prior art techniques of both indirect and direct squeezecasting.

Indirect squeeze casting is generally considered to be a modification ordevelopment of high pressure die casting. Liquid metal is forced into aclosed die cavity from a shot sleeve by a small piston. The piston,normally driven by a hydraulic ram continues to act on the metal in thedie cavity during its solidification period. Squeeze pressures arelimited by the bore of the piston and the rating of the hydraulic ram.In one method of indirect squeeze casting liquid metal is pouredturbulently into a shot sleeve and the metal is non-turbulentlydisplaced upwards into the die cavity by the piston through a gate, thewidth of the gate being many times larger than the gate used inconventional high pressure die casting. The die may open eithervertically or horizontally to release the casting. In a second method ofindirect squeeze casting, metal is injected turbulently at high velocityinto the die cavity through a narrow gate and the metal is furtherconsolidated in the mould by means of opening the gate wider to allowthe piston to move forward to compensate for solidification shrinkage.

Indirect squeeze casting processes generally operate with short cycletimes since they are generally based on high pressure die castingpractice and hence they tend to be high productivity processes. By thesame token, however, these processes are most suited to conventionalcasting alloys and it has been found difficult to manufactureconsistently good castings from high strength aluminium wrought alloys.Even those indirect squeeze casting produced using conventional castingalloys may contain some remnant microporosity in regions remote from theaction of the plunger which could be detrimental to the quality of thecastings. Furthermore, in most indirect squeeze casting practices it isnot feasible to filter the metal just prior to its entry into the diecavity and therefore the oxides present in the melt due to turbulentmetal handling procedures inevitably become trapped in the cast articleas non-metallic defects. Such defects undermine and diminish the qualityof the cast metal product to an extent that cannot be easily quantifiedand which cannot be readily tolerated.

With regard to metering metal to produce castings of constant size andshape, indirect squeeze casting processes use a closed die to define thecasting cavity. Opposing die halves are locked together rigidly byhydraulic cylinders and large toggle during the injection of metal fromthe shot sleeve and during the metal pressurisation period from theplunger. The only moving part of the equipment during casting is theplunger, which moves to inject metal into the fixed volume die cavity.Side cores may be used in the die to increase the complexity of thecasting shape. Although the external form of the cast article may beaccurately controlled, indirect due to the presence of microporosity,particularly in regions of the casting remote from the plunger. Excessmetal comprising the runners and wad, which may constitute over fiftypercent of the total shot mass, needs to be removed from the castingafter ejection from the die.

Of the four casting conditions listed above as pre-requisites for thehighest quality castings, indirect squeeze casting only satisfies thethird requirement, namely, the accurate metering of metal into the diecavity.

Direct squeeze casting differs from indirect squeeze casting in severalimportant ways. In the prior art direct squeeze castings are generallymade in a vertically acting hydraulic press. Liquid metal is poured froma spoon or robotic ladle or down a launder into a lower die cavitysituated on the lower platen of the hydraulic press and the top part ofthe die is lowered by means of movement of the upper platen into thelower die cavity to displace liquid metal so as to fill the entire diecavity. Pressure from the hydraulic press continues to act on the metalin the mould during its solidification period by the continued movementof the top part of the die, or punch, into the lower part of the dieassembly, or lower die cavity. The pressure on the casting duringsolidification is governed only by the working capacity of the hydraulicpress. No runners or risers are required and the direct squeeze castingprocess is extremely efficient in metal utilisation for near net shapecastings.

The major advantage of direct squeeze casting over indirect squeezecasting is the application of pressure over the entire or larger part ofthe surface area of the casting by the movement of the punch within thelower die half. Because of this relative movement of the two die halves,the cast metal is very effectively squeezed throughout its freezingperiod and is pressurised even in the remotest regions of the casting.Hence, liquid metal fluidity is not a prime requirement of directsqueeze casting and alloys other than conventional casting alloys may beused. Forging alloys, metal matrix composites of particulate ingot andpreform infiltration varieties, together with other "difficult to cast"alloys, in addition to conventional casting alloys, have all been castusing the direct squeeze casting route.

In the prior art of direct squeeze casting, liquid metal has invariablybeen fed into the lower die cavity from above in a turbulent fashion.Although filters can be placed, for instance, in the path of the metalstream as it travels down a launder system, the metal finally enteringthe die will inevitably do so in a turbulent manner and willconsequently generate more oxide films which become engulfed in thecasting.

Another problem, concerned with metal metering, also stems from the factthat the two halves of the die set are, of necessity, set widely apartinitially in order to allow liquid metal ingress. In such circumstancesor die filling, the only means to date of metering metal into the cavityhas been by the timing of metal flow from a dosing furnace or by usingladles of a given volumetric capacity. In both cases, variations incasting mass may occur, leading to variations in the through thicknessof the direct squeeze castings which may place the castings outsideacceptable tolerance limits. One way of addressing this problem has beento allow excess metal to flow out of the die set through gaps or windowsbetween the punch and the lower half as the punch enters the lower halfprior to pressing. This procedure has not been found to be a completelysatisfactory solution to the problem and it has not been widely adoptedin practice.

Thus, direct squeeze casting as practised currently can be seen tosuffer from serious limitations and satisfies only one of thepre-requisites listed above for the highest integrity castings, namely,the full pressurisation of the cast metal during solidification. In-linefiltration can also be achieved if an appropriate launder system is usedbut it cannot be implemented for the commoner practices using roboticladling of liquid metal into the die set. It will be appreciated thatdirect squeeze casting as used in the prior art tends to be a cumbersomeprocess combining, as it does, gravity die casting with closed dieforging. The resultant questionable cast metal quality together with aninherently low process productivity have restricted the industrialapplication of direct squeeze castings.

Therefore, there exists a need for a high productivity direct squeezecasting process which is capable of producing the highest quality nearnet shape castings in high strength alloys at an economically viableproduction rate.

In one respect the invention is an apparatus for casting metal articlescomprising a receptacle for molten metal, and co-operating upper andlower die parts which define at least one mould cavity for casting themetal article. The die parts are movable with respect to each other, andtheir separation distance is selected to define a predetermined cavityvolume for the cast article. Pressurising means are provided to applypressure on the die parts to further reduce the cavity volume duringsolidification of the metal in the mould cavity. A conduit has a firstend connected to an entrance in the lower die part and a second endconnected to the receptacle. Means are provided for transferring moltenmetal upwardly from the receptacle through the conduit to fill orsubstantially fill the mould cavity in a non-turbulent manner. Sealingmeans which seals the entrance to the lower die part is below the lowerdie part and includes a sliding gate located between the first end ofthe conduit and the entrance in the lower die part.

By these means, squeeze castings free or substantially free fromporosity can be produced to near net shape in a process that can beoperated repetitively at high production rates to produce high qualityproducts at acceptable cost.

Preferably, the apparatus further comprises opening means to open themould.

Preferably, the apparatus comprises extraction means to remove the castarticle from the mould cavity.

Preferably, the receptacle for molten metal is a heatable furnace.

Preferably, the receptacle for molten metal is an unheated reservoir.

Preferably, each die part is supported on a platen of which at least oneplaten is slidable one or more tie bars.

Preferably, the means for transferring molten metal in the conduit is alow pressure pneumatic system.

Preferably, the means of transferring molten metal in the conduit is anelectromagnetic pump.

Preferably, the means of transferring molten metal in the conduit is avacuum system to create a negative pressure in the mould relative to thereceptacle.

Preferably, the sliding gate is manufactured from an inert materialcapable of forming a leak-tight closure.

Preferably, the sealing means comprises a slide track such that the dieparts are slidable upon it to enable the entrance to the mould cavity tobe taken out of feeding relationship with the conduit.

Preferably, the slide track is manufactured from an inert materialcapable of forming a leak-tight closure.

Preferably, the sealing means comprises a stopper located within themould cavity above the entrance to the mould cavity which can be loweredto seal the entrance to the mould cavity.

Preferably, the pressure applied by the pressurising means duringsolidification progressively deforms and compresses the solidified metalin the mould to compensate for contraction during solidification toensure the removal or substantial removal from the casting ofcontraction cavities or remnant gas porosity from gases dissolved in themetal.

Preferably, the pressurising means has a variable speed of operation.

Preferably, monitoring means are provided to monitor the pressureapplied and the displacement produced by the pressurising means and togenerate a specific pressurisation and/or displacement regime.

Preferably, the apparatus further comprises a filtration means to filterthe molten metal prior to entering the mould cavity.

Preferably, there are a plurality of mould cavities.

Preferably, the lower die part is encased in a steel bolster.

Preferably, the upper die part is held on a steel backing plate orsupport block.

Preferably, the die parts are provided with heating/cooling means.

Preferably, the separation of the die parts is determined bydisplacement transducers.

Preferably, the separation of the die parts is determined bycompressible separators.

Preferably, the pressurising means is a hydraulic press.

Preferably, the die parts comprise impression blocks manufactured in oneor more interlocking segments from hardened and tempered steel.

Preferably, the surfaces of the mould cavity are coated with a lubricantor release agent.

The invention also involves a method of casting metal articles, whereina mould which has at least one mould cavity of variable volume islocated above a receptacle which contains molten metal. A conduit has afirst end connected to an entrance in the mould and a second endconnected to the receptacle. Molten metal is forced upwardly from thereceptacle through the conduit and into the mould cavity withoutturbulence. The loss of molten metal from the mould cavity is preventedby sealing the entrance to the mould with a sliding gate which is movedfrom an open position which is below the mould cavity to a closedposition which is also below the mould cavity. During solidification ofthe molten metal, pressure is applied to the mould cavity to reduce itsvolume, thereby compensating for contraction during solidification.

Preferably, the pressure is reduced after solidification is completed.

Preferred embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings ofwhich:

FIG. 1 shows the general construction of the direct squeeze castingapparatus;

FIG. 2 shows an enlarged view of the mould and sliding gate assembly inFIG. 1;

FIG. 3 shows enlarged details of the sliding gate and the upwardlyacting locking mechanisms in FIG. 2;

FIG. 4 illustrates a preferred metering mechanism using compressibleseparators with a sliding mould;

FIG. 5 illustrates an alternative metering mechanism with a slidingmould and punch; and

FIG. 6 shows an alternative sealing mechanism.

FIG. 1 shows the general assembly of direct squeeze casting equipmentcomprising the hydraulic press 1 with its control/operations panel 2 andthe pumping furnace 3 located approximately centrally below the lowerplaten 4 of the hydraulic press 1. The control/operations panel 2includes means for monitoring the pressure applied and the displacementsproduced by the press 1 during solidification, and to generate aspecific pressurisation and/or displacement regime. In this embodimentthe lower die half 5 which may itself be constructed of separatesegments which define the form of the lower surfaces of the casting andwhich may be held together in a robust steel bolster, is held rigidly onthe lower platen 4 by bolts, jacks, levers or the like. The punch orupper die half 6 which may itself be constructed of separate segmentsand which defines the form of the upper surfaces of the casting, isattached directly or indirectly via a backing plate or a support blockto the upper platen 7 above the lower die half and can be lowered intoand raised from it to open the mould be means of the operation of thehydraulic cylinder or cylinders 8 and the associated hydraulic pumps 8aof the hydraulic press 1. The die parts 5,6 and any cores which may bepart of the die construction may be sprayed with a graphitic or otherdie lubricant prior to assembly in their correct locations ready forcasting. The upper part of the die 6 is positioned, with the aid ofdisplacement transducers, limit switches or other similarly suitabledevices, within the lower part of the die 5 to define a metal meteringmeans. The pumping furnace 3 holds a supply of molten metal in areceptacle and transfers the molten metal upwardly through a conduit 9to the mould. The metal metering means is such that the volume of liquidmetal which will enter the cavity from the conduit 9 will, subsequent tosealing the entrance to the mould and after solidification andsimultaneous compression and compaction by the main free orsubstantially free from porosity and of the required dimensions. Usuallythe dies are operated at a temperature within the range 250° C.-350° C.and they may be heated or cooled to maintain their means. The heatingmay be applied directly to the impression blocks or to the bolster andsupport block of the upper an lower parts respectively.

FIG. 2 shows an enlarged view of a die set 5 and 6 located centrally onthe platens 4 and 7 above the conduit 9, which comprises a riser tube 9afrom the furnace 3 and a channel 9b in the lower platen, with thesliding gate 10 situated between the top of the conduit and the bottomof the die cavity 11, the sliding gate being operated by the hydrauliccylinder 22. The orifice 12 in the sliding gate 10, which may be linedwith ceramic or other inert material, lines up with the conduit 9 whenthe gate is open to allow metal to pass upwardly from the conduit 9 intothe mould cavity 11 through a filtering medium 13 situated within theorifice 12 of the sliding gate 10 or at any suitable position within themould entrance 14. The channel 9b in the lower platen 4 and in anybacking plates between the lower die half 5 and the lower platen 4 maybe lined with inert ceramic material 15. When the correct amount ofmetal has passed into the mould cavity the sliding gate 10 is moved by adistance greater than the diameter of the orifice 12 to displace theorifice 12 in the sliding gate 10 fully out of alignment with theconduit 9 and thus to isolate the liquid metal in the mould cavity 11from the metal in the conduit 9. At this stage pressure can be reducedin the pumping furnace 3 to allow the descent of liquid metal in theconduit 9. To facilitate the removal of liquid from beneath the undersurface of the sliding gate 10, the underside of the sliding gate 10 maycontain a venting passage to allow the ingress of air into the top ofthe conduit 9 when the venting passage and the inner edge of the conduitare placed in juxtaposition by the further movement of the sliding gate10. The contacting surfaces above and below the sliding gate may be of amaterial different from the sliding gate 10 to reduce frictional effectsand to prevent any potential seepage of liquid metal into the slidingmechanism.

The sliding gate 10 can be secured in its unaligned position to form aleak-tight seal by forcing it from below against the lower end of themould entrance 14. In FIG. 3 the upwardly acting force on the slidinggate 10 may be generated from the interaction of the inclined surfaces16 in the sliding gate against the inclined ramps 17 on the underlyingsurface. The mechanical or hydraulic action of levers or toggles or thelike may also be used to effectively seal the sliding gate 10 againstthe lower end of the mould entrance 14. Once a lead-tight seal has beenformed the pressure of the hydraulic ram 8 acting through the punch 6can be brought to bear on the metal in the cavity 11 throughout thesolidification period. When the casting has solidified any side corescan be withdrawn, the punch 6 can be retracted by the reverse action ofthe main cylinder or cylinders 8 or by the action of ancillary cylindersand the casting can be removed. The small disc of solidified aluminiumin the orifice 12 in the sliding gate 10 can easily be removed by asprung ball mechanism, for instance, and the die set can be reassembledfor further casting. This method of operation is suitable for the largersize of castings in heavy steel moulds and bolsters. This arrangement ofdie filling and sealing is suitable, for example, for the manufacture ofautomotive components such as steering knuckles, wheel hub castings,light alloy wheels and other shapes for general engineeringapplications.

An alternative arrangement for metal metering and mould sealing is shownin FIG. 4, in which the bottom half of the die set 5 is situated on araised or recessed slide track 18 and is moved with respect to theconduit 9 to place the mould into or out of feeding relationship withthe furnace 3 and simultaneously out of or into coaxiality with thehydraulic press 1. In this embodiment the mould entrance 14 is initiallylocated over the conduit 9 and metal enters the mould from below in anon-turbulent manner through a filtering mechanism 13 located in theliquid metal flow path below the mould entrance 14 or at any suitableposition within the mould entrance 14. Metering of the correct amount ofliquid metal into the mould is effected through having the top half ofthe die 6 situated within the lower half of the die 5 so defining themould cavity 11 but raised on, preferably adjustable, compressibleseparators 19 by an amount which just compensates for the volumecontraction of the liquid metal on solidification and compaction by thesqueezing forces of the hydraulic press 1.

Instead of being situated on compressible supports 19 as shown in FIG. 4the top half of the die 6 may alternatively be held at the requiredmetal metering height on its own slide track 18 attached directly orindirectly to the top platen and directly above the lower slide track asshown in FIG. 5. The synchronous sideways sliding movement of the twohalves of the die may be actuated by a single hydraulic cylinder actingin the line of the slide tracks with the punch 6 being moved byinteraction with the lower half 5, or vice versa, or by the cylinderacting jointly on the two die halves; or by a pair of cylinders actingsimultaneously on the two halves.

After filling the mould with the requisite amount of liquid metal ineither of the embodiments shown in FIGS. 4 and 5, the top half 6 and thebottom half 5 of the mould are together moved sideways out of feedingrelationship with the conduit 9 to a position near the centre of theplatens of the hydraulic press. At the end of this movement the punch 6in FIG. 4 can engage securely in a seating such as a tapered keyway orsome other interlocking or interacting device on the underside of theupper platen 7 in order to create a withdrawal mechanism for the punchwhen the casting has solidified. Preferably, pressure is brought to bearon the bottom half of the mould by ancillary hydraulic or pneumaticpistons or by mechanical devices to seal the entrance to the mould bymaking a leak-proof joint between the die and the lower slide track.Pressure from the hydraulic press is then applied to the punch 6 of thedie set such as to displace the punch to compensate for solidificationcontraction until the liquid metal fully solidifies. The die set canthen be opened and the casting removed.

The arrangements of die filling and sealing described in the embodimentsshown in FIGS. 4 and 5 are suitable, for instance, for pistonmanufacture for automotive and other applications which may be ofmonolithic light alloy composition or metal matrix compositeconstruction, although other automotive and general engineeringcomponents are also suitable for this process.

Another alternative way of sealing the mould is illustrated in FIG. 6 inwhich the moving part 21 of the sealing means is held in the upper partof the die 6. After being coated with the necessary die lubricant thedie halves are initially set at the appropriate distance apart such thatthe correct quantity of liquid metal is metered into the die cavity 11when the die cavity is filled from below. The moving part 21 of thesealing means can then be lowered by actuation of an ancillary hydrauliccylinder 23 or by pneumatic or mechanical devices through the liquidmetal to seat on the entrance to the mould 14 in the lower half of thedie 5 and to effectively seal it to prevent the flow of liquid from themould during the pressurisation cycle of the hydraulic press. Pressurecan be exerted on the moving part 21 of the sealing means by theancillary hydraulic cylinder 23 or by other pneumatic or mechanicaldevices to ensure that a pressure tight joint exists to prevent the lossof metal from the mould. Once the seal has been made secure thesqueezing pressure from the main hydraulic cylinder 8 can be applied toconsolidate the casting during solidification. After solidification themoving part 21 of the sealing means can be withdrawn by reverseactuation of the ancillary hydraulic cylinder 23 or pneumatic piston orby release of the mechanical devices, the die set can be opened usingthe return action of the main hydraulic cylinder or cylinders 8 and thecasting can be removed from the mould cavity. This embodiment of thedirect squeeze casting process is particularly suitable for componentswhich require a through hole such as steering knuckles and wheels orwheel centres, for instance, and for moulds containing multiple cavitiesarranged around a common ingate.

In the general procedures described above, the punch 6 is located in itsmetal metering position prior to metal entering the die. However thelowering of the punch 6 to its metering position in the lower half ofthe die 5 to define the die cavity can occur simultaneous with mouldfilling, or even subsequent to mould filling but prior to mould sealing,in order to promote enhanced liquid metal movement in the die cavity andto encourage more refined microstructure in the cast article. Such aprocedure will also provide shorter manufacturing times and greaterproductivities, particularly for large volume castings.

By following the manufacturing routes described, a range of articles canbe produced which are near net shape and contain little or no porosity.By placing filers in-line with the metal flow large oxide particles andother deleterious solid inclusions may be removed from the casting andhigh integrity castings are produced. By this process conventionalcasting alloys and alloys of conventional and non-conventional forgingcomposition may be successfully cast into products close to final form.Local reinforcement of the castings can also be effected by theplacement of ceramic or metallic preforms at selected locations in thecavity which are fully infiltrated during casting to produce metalmatrix composite regions having enhanced properties.

Although reference has been made specifically to light alloys, it shouldbe noted that other non-ferrous and ferrous alloys, particulate metalmatrix composites and semi-solid alloys, may also be squeeze cast usingthe method and apparatus described. For the high temperature alloys, diesets constructed, at least in part, of heat resistant ceramic materialssuch as sialons may have to be used.

I claim:
 1. An apparatus for casting metal articles comprising areceptacle for molten metal, at least one mould cavity for casting themetal article, the mould cavity being defined by co-operating upper andlower die parts, the die parts being movable with respect to each otherand their separation distance being selected to define a predeterminedcavity volume for the cast article, a conduit having a first endconnected to an entrance in the lower die part of the mould cavity and asecond end connected to the receptacle, means for transferring moltenmetal upwardly from the receptacle through the conduit to fill orsubstantially fill the mould cavity in a non-turbulent manner, sealingmeans being provided to seal the entrance to the lower die part, whereinpressurising means are provided to apply pressure on the die parts tofurther reduce the cavity volume during solidification of the metal inthe mould cavity, the sealing means being below the lower die part andcomprising a sliding gate located between the first end of the conduitand the entrance in the lower die part.
 2. An apparatus as claimed inclaim 1, further comprising opening means to open the mould.
 3. Anapparatus as claimed in claim 1, wherein the receptacle for molten metalis a heatable furnace.
 4. An apparatus as claimed in claim 1, whereinthe receptacle for molten metal is an unheated reservoir.
 5. Anapparatus as claimed in claim 1, wherein each die part is supported on aplaten of which at least one platen is slidable on one more tie bars. 6.An apparatus as claimed in claim 1, wherein the means for transferringmolten metal through the conduit is an electromagnetic pump.
 7. Anapparatus as claimed in claim 1, wherein the sliding gate ismanufactured from an inert material capable of forming a leak-tightclosure.
 8. An apparatus as claimed in claim 1, wherein the sealingmeans comprises a slide track such that the die parts are slidable uponit to enable the entrance to the mould cavity to be moved away from theconduit.
 9. An apparatus as claimed in claim 1, wherein the slide trackis manufactured from an inert material capable of forming a leak-tightclosure.
 10. An apparatus as claimed in claim 1, wherein the pressureapplied by the pressurising means during solidification progressivelydeforms and compresses the solidified metal in the mould to compensatefor contraction during solidification and to ensure the removal orsubstantial removal from the casting of contraction cavities or remnantgas porosity from gases dissolved in the metal.
 11. An apparatus asclaimed in claim 1, wherein the pressurising means has a variable speedof operation.
 12. An apparatus as claimed in claim 1, wherein monitoringmeans are provided to monitor the pressure applied and the displacementproduced by the pressurising means during solidification and to generatea specific pressurisation and/or displacement regime.
 13. An apparatusas claimed in claim 1, further comprising a filtration means to filterthe molten metal prior to entering the mould cavity.
 14. An apparatus asclaimed in claim 1, wherein there are a plurality of mould cavities. 15.An apparatus as claimed in claim 1, wherein heating/cooling means areprovided for the die parts.
 16. An apparatus as claimed in claim 1,wherein the separation of the die parts is determined by displacementtransducers.
 17. An apparatus as claimed in claim 1, wherein theseparation of the die parts is determined by compressible separators.18. An apparatus as claimed in claim 1, wherein the pressurising meansis a hydraulic press.
 19. A method of casting metal articles comprisingthe steps of locating a mould which has at least one mould cavity ofvariable volume above a receptacle containing molten metal, connecting aconduit between the mould cavity and the receptacle, said conduit havinga first end connected to an entrance to the mould and a second endconnected to the receptacle, forcing molten metal upwardly from thereceptacle through the conduit and into the mould cavity withoutturbulence, preventing loss of molten metal from the mould cavity bysealing the entrance to the mould with a sliding gate which is movedfrom an open position which is below said mould cavity to a closedposition which is also below said mould cavity, wherein duringsolidification of the molten metal pressure is applied to the mouldcavity to reduce the volume thereby compensating for contraction duringsolidification.
 20. A method as claimed in claim 19, wherein thepressure is reduced after solidification is completed.
 21. A method asclaimed in claim 20, wherein the mould is then opened and the castarticle is removed from the mould.